Willi A. Ribi

Distribution and properties of sex-specific photoreceptors in the flyMusca domestica

Summary1.In male houseflies (Musca domestica) the frontal dorsal region of the eye contains a unique class of central rhabdomere (R7/8) not found in other eye regions or in female flies (Fig. 1). The rhabdomeres may be recognised in vivo by their red autofluorescence, and are called here 7r and 8r respectively.2.Difference spectra of 7r rhabdomeres, measured by microspectrophotometry of single rhabdomeres are indistinguishable from those of R1–6 (Fig. 2).3.Intracellular recordings coupled with dye injections have established that: a) 7r cells are indistinguishable from the peripheral photoreceptors R1–6, at least with respect to spectral, angular and absolute sensitivities, response waveform and noise characteristics (Figs. 4, 5; Table 1); b) 8r cells however are clearly distinguishable by virtue of their spectral sensitivity (Fig. 6), noise characteristics and response waveform (Fig. 5).4.Axonal profiles from cells stained intracellularly with the dye Lucifer yellow (Fig. 9) show that: a) 7r cells do not project to the medulla but terminate in the upper region of the lamina cartridge layer where they also project out one or more lateral branches; b) 8r cells project long axons through to the medulla.5.Electron microscopic examinations of cells initially identified by their autofluorescence indicate that 7r cells approximate many features of R1–6 cells, including cell body, rhabdomere and axonal diameters. In these respects 8r cells differ and show the characteristic morphology of other R7 and R8 cells (Fig. 8, Table 2).

The synaptic organization of visual interneurons in the lobula complex of flies

SummaryThe synaptic organization of three classes of cobalt-filled and silver-intensified visual interneurons in the lobula complex of the blowfly Calliphora (Col A cells, horizontal cells and vertical cells) was studied electron microscopically. The Col A cells are regularly spaced, columnar, small field neurons of the lobula, which constitute a plexus of arborizations at the posterior surface of the neuropil and the axons of which terminate in the ventrolateral protocerebrum. They show postsynaptic specializations in the distal layer of their lobula-arborizations and additional presynaptic sites in a more proximal layer; their axon terminals are presynaptic to large descending neurons projecting into the thoracic ganglion. The horizontal and vertical cells are giant tangential neurons, the arborizations of which cover the anterior and posterior surface of the lobula plate, respectively, and which terminate in the perioesophageal region of the protocerebrum. Both classes of these giant neurons were found to be postsynaptic in the lobula plate and pre- and postsynaptic at their axon terminals and axon collaterals. The significance of these findings with respect to the functional properties of the neurons investigated is discussed.

The second and third optic ganglia of the worker bee

SummaryThe gross morphology and the fine-structural characteristics of neurones of the second and third optic ganglia of the honeybee Apis mellifera were investigated light microscopically on the basis of Golgi (selective silver)- and reduced silver preparations.The second optic ganglion, the medulla, is ovoid in shape and has a slightly convex distal surface and a slightly concave proximal surface. The medullar outer levels are characteristically composed of neuronal arrangements showing strict precision of their geometrical spacing proximally as far as a pronounced layer of tangential fibre elements comprising the serpentine layer of the medulla. At the inner medullary levels retinotopic channels are again multiplied, and the arrangement of axons and dendrites contribute to a complex lattice.The third optic ganglion, the lobula, is interposed between the medulla and the protocerebrum. It is the site of termination of the third-order neurones. The lobula in hymenopterans appears, in contrast to dipterans, odonates and lepidopterans, as a single neuropilic mass.A short review of the electrophysiological data concerning these two ganglia has been tentatively correlated with some of the anatomical data.

Coloured screening pigments cause red eye glow hue in pierid butterflies

SummaryThe fine structure of the tracheal tapetum lucidum was investigated in six members of the diurnal lepidopteran family Pieridae. These observations were compared with the eye glow hue on intact eyes, the reflection colour of the incident illuminated tapetum and the distribution of the red screening pigments found in some receptor cells.The eye glow hue of dark adapted pierid eyes appears dark red. Whereas in the speciesColias crocea, C. australis andGonepteryx rhamni the red eye glow is observed all over the eye, appear the uppermost 35 horizontal eye rows in the speciesPieris brassicae, P. napi andP. rapae turquoise to yellow-green. The reflection colour of the tracheal tapetum of all examined pierid eyes is, however, uniformly turquoise to yellow-green all over the eye. In addition no regional structural differences in the tracheal tapetum at the proximal end of the rhabdom were observed. One knows, however, that inPieris brassicae, P. napi andP. rapae in the ventral and medial part of the eye, inColias crocea, C. australis andGonepteryx rhamni in all ommatidia the four proximally located receptor cells 5–8 contain red screening pigment which is arranged close to and even within the rhabdomeric structures (Ribi, 1978a). These observations show that the red eye glow observed in various pierid butterflies depends on coloured retinula cell screening pigment and not on the tracheal and rhabdomeric structures. Morphological and functional aspects of the Lepidopteran tapetum lucidum and the role of the coloured retinula cell pigmentation are discussed.

Anatomical identification of spectral receptor types in the retina and lamina of the Australian orchard butterfly, Papilio aegeus aegeus D.

SummaryThe nine receptor cells examined in each ommatidium of the butterfly Papilio aegeus aegeus can be named according to their positional orientation across the fused rhabdom. Six of them end as short visual fibres (svf) in the second stratum of the lamina, whereas the remaining three retinula cells (lvf) pass together with the lamina fibres (L-fibres) the first optic ganglion and the outer chiasma to end in the three most distal layers of the second optic ganglion, the medulla. The organization of the retinula-cell axons within the pseudocartridge and the cartridge remains almost uniform throughout the first optic ganglion. Five L-fibres, which have their origin in the fenestrated layer (FL), join each laminar cartridge before entering the neuropil of the first optic region. Four of these L-fibres (L-1, L-2, L-3 and L-4) could be definitely located and characterized using Golgi-stained light- and electron-microscopic techniques. Whereas L-1 and L-3 show a lateral branching pattern reaching only fibres of the same cartridge, L-2 and L-4 have long collaterals interconnecting several neighbouring cartridges in a characteristic pattern. Serial sections of silver-impregnated retinula-cell axons as well as L-fibres were investigated for their synaptic connectivity patterns between and within these fibres. These cellular interactions and possible information processing are discussed.

Do the rhabdomeric structures in bees and flies really twist

SummaryElectron microscopic observations following careful tissue treatment show that the microvilli in visual cells of both the bee and the fly do not twist (Figs. 1 and 2). Theories of how bees analyse polarized light are reexamined in the light of this finding.

The Structural Basis of Information Processing in the Visual System of the Bee

The optic lobes of the bees are composed of three ganglia. From the periphery inward, the three regions are known as the lamina, the medulla and the lobula. The three optical ganglia are connected to each other by the outer chiasma which connects the lamina with the medulla and the inner chiasma which connects the medulla with the lobula (Fig. 1). The first optic ganglion, the lamina, is a neuropil zone consisting of thousands of cylindrical units, the optical cartridges, which receive the inputs of the receptor cell axons (RCA’s). The nine RCA’s emerging from the ommatidium, project in one bundle either as short visual fibers (svf) or long visual fibers (lvf) to the cartridge below. In addition to the RCA’s and the four different L-neurons in each cartridge an unknown number of tangential, centrifugal, and horizontal fiber elememts are found. The medulla, like the retina and lamina, shows in its distal layers a highly regular arrangement of axon bundles. The visual information reaches the medulla through at least seven channels either directly from the retina via the three long visual fibers or after relay from the six short visual fibers via the four L-fiber types. The main neuronal elements in the medulla are the transmedullary cells and an unknown number of tangential and amacrine cells. Y- and T- cells link the medulla with the third optic ganglion. The lobula is a multi-stratified, spherical-shaped ganglion. The three outermost strata contain endings of the transmedullary cells and those of the shallow endings of Y- and T-cells. The neuronal endings leave the ganglion in several fibre bundles and project to the posterior or anterior protocerebrum.

New aspects of polarized light detection in the bee in view of non-twisting rhabdomeric structures.

SummaryPolarization orientation in the bee is limited to short wavelengths (Frisch, 1967; Heiversen and Edrich, 1974). Although each ommatidium contains three UV-receptors (two long and a short retinula cell) only the short and proximally located ninth retinula cell has a high sensitivity to the plane of polarized light (Menzel and Snyder, 1974).A map over the dorsal eye of the microvillar orientations of the ninth cells was constructed using special fixation techniques that results in minimum distortion of rhabdomeric structures (Ribi, 1979).In a region between the 5th and the 45th horizontal ommatidial row (counted from the top) three ninth cell types (X, V, Y) according to their microvilli directions, were found. One third of 119 observed ninth cells (type X cells) have their microvilli arranged 52°±7° S.D. to the z-axis. A second third of ninth cells (type V cells) have their microvilli 89°±7° S.D. to the z-axis whereas the remaining third of ninth cells (type Y cells) have their microvilli 127°±7° S.D. to the z-axis.The analysis of polarization information based on simultaneous measurements requires at least three inputs for an unambiguous e-vector detection (Kirschfeld, 1972). The pattern of ommatidia as described here is such that three adjacent ommatidia always have ninth cells with three different microvilli directions. It is suggested that such a triplet of ommatidia represents a unit capable of unambiguous e-vector detection.

Light and electron microscopic structure of golgi-stained neurons in the vertebrate brain (new rapid Golgi procedure)

SummaryAfter application of a rapid, selective silver impregnation procedure for light (LM) and electron (EM) microscopy, individual neurons are distinguishable by a light silver precipitation. The silver content is sufficient that entire nerve cells can be observed light microscopically; on the other hand, electron microscopically the cytological details are still visible. Brains of mice were fixed by phosphate-buffered aldehyde perfusion, and pieces of tissue left in a 1 % K2Cr2O7 solution for 13 h before impregnation in a 0.5 % AgNO3 solution for 2h. Thick sections (30–50 μm) of the impregnated tissue were cut; from these sections, suitably stained neurons were dissected out and re-embedded for ultrathin sectioning, thereby allowing observations on the same neurons at the EM level. A thin silver deposit was observed along the delimiting neuronal membrane, the microtubules and the smooth ER, including the spinal apparatus of the dendritic spines. The fine cytoplasmic details of the impregnated neurons and the surrounding tissue are well preserved and, therefore, suitable for subsequent determination of synaptic relationships of the impregnated neurons with the adjacent neuronal elements.

Electron Microscopy of Golgi-Impregnated Neurons

This chapter will describe and discuss a combined Golgi—electron microscopy (EM) technique that successfully preserves gross morphology and ultrastructure, especially the synaptology, of identified neurons. In addition, various combinations of fixation, chromation and impregnation useful for insect and vertebrate nervous tissue are described. The Appendix lists schedules for fixation and chromation, giving impregnation times.